Spotlight on the Eastern Black Swallowtail

Chances are, if you grow any plant from the Carrot family (parsley, rue, celery, fennel, dill, golden alexander etc.) then may have seen this caterpillar eating on your plants or butterfly flitting around your garden!

Some people regard them as "worms" or "pests" but the truth is, they are important pollinators and the caterpillars that will turn into a beautiful butterfly one day, the Black Swallowtail (Papilio polyxenes)!

Read on to learn about these fascinating butterflies and how to live harmoniously with them in your garden!

Eastern Black Swallowtail nectaring on a zinnia. Photo by Rebecca Chandler

Among the favorite nectar plants of the Eastern Black Swallowtail are milkweed, zinnias, native thistles, cone flowers, sunflowers and many others. 

Monarch vs Black Swallowtail

People often confuse the Black Swallowtail caterpillar with Monarch caterpillars, especially when they look the most similar, in their 5th Instar. There are some main differences to look out for.

​The Black Swallowtail caterpillars have thicker, green stripes and yellow, wavy "dots". They also don't have the long tentacles on their head and tail end like Monarch caterpillars do.

​The most telltale sign is that Monarch will be eating on Milkweed while Eastern Swallowtails will be nibbling on plants of the Carrot family (dill, fennel, rue, Queen Anne's lace, celery, parsley etc.)

Host Plants of the Eastern Black Swallowtail

Adult Eastern Black Swallowtails will lay eggs on plants in the Carrot family. You may find the caterpillars eating on these plants in great numbers but don't be alarmed! There are several ways to keep the caterpillars happy while also saving some of the plant for yourself!

How to keep the caterpillars happy and not lose all of your plants or resort to using pesticides!

1. Plant extra dill or fennel so that there is enough for them and you to eat.
2. Pick the caterpillars off of the plant and put them into a butterfly enclosure with the host plant to watch them pupate and eclose into a beautiful butterfly.
3. Successional planting of dill/fennel/parsley etc. throughout the summer so there is always enough to go around. ​

Lifecycle of the Black Swallowtail 

Similar to the Monarch butterfly: The female butterfly lays one yellow egg at a time on a host plant. After 4-9 days, a caterpillar (larva) hatches from the egg. Over 10-30 days, the caterpillar grows through 5 different stages (called instars) to a length of 2 inches.

Male vs Female

The male features a large row of yellow-colored spots across the middle of its wings. The female has smaller spots, but a larger area of blue scales on the lower wings.

Female Black Swallowtail (Top) with more blue visible. Male Black Swallowtail (Bottom) with more yellow visible. Photo by Rebecca Chandler

FUN FACT

When provoked, Black Swallowtail caterpillar will emit a disagreeable odor through their osmeteria, a V-shaped orange protrusion on their heads!